An image quality correction circuit using histogram illustrating distribution of television signal levels is disclosed in the Japanese Laid-open Patent No. H5-66751. FIG. 12 shows a block diagram of an image quality correction circuit of the aforementioned patent.
The image quality correction circuit comprises: a cumulative histogram circuit 81 for creating a histogram for each field of an input video signal and calculating a cumulative histogram from the created histograms, a first multiplier 82 for multiplying the output of the cumulative histogram circuit by a first constant, a normalization circuit 83 for dividing the output of the first multiplier 82 by the total number of samples, a correction circuit 84 for outputting a correction value for each picture element using the output of the normalization circuit 83 and the input video signal, a second multiplier 85 for multiplying the output of the correction circuit 84 by a second constant, and an adder 86 for adding the output of the second multiplier 85 and the input video signal. The image quality correction circuit as configured above virtually improves the contrast without depending on the performance of television sets by increasing the assignment of brightness on television sets corresponding to the interval of the most concentrated signal levels. This is accomplished by modifying the frequencies of the histogram of the input video signal.
The above configuration for an image quality correction circuit may result in excessive correction, because the histogram generated from the input video signal and unmodified is used. The generated histogram may be biased if the input video signals are concentrated near a certain level causing the slope of the cumulative histogram to be steep near this level. The steep slope affecting, in some cases, excessive correction such as intensified noise or unnatural images